CNN — Two astronauts conducted a spacewalk Saturday to address an ammonia leak in the International Space Station's cooling system, a mission that ended with NASA optimistic the potentially major problem had been fixed.

In addition to inspecting the site of the leak, NASA astronauts Chris Cassidy and Tom Marshburn removed and replaced a 260-pound pump controller box, NASA explained.

Afterward, Cassidy and Marshburn couldn't detect any sign of a leak -- which might have been manifest, in this case, as ammonia snow flakes -- when space agency officials back on Earth ran the new pump. Still, NASA noted that the area needs to be monitored long-term to prove that the problem has been fixed.

"No leaks!" tweeted Chris Hadfield of Canada, the space station's commander, who choreographed the spacewalk. "We're bringing Tom & Chris back inside. ... This is an amazing place & time."

The entire spacewalk took 5 hours and 30 minutes -- an hour less than expected.

First detected early Thursday morning, the leak was causing ammonia to enter space at a rate of 5 pounds per day, Mike Suffredini, NASA's space station program manager, told reporters Friday. Ammonia is used to cool the solar arrays that provide electricity to station systems.

The discovery spurred teams at NASA, over a busy 24-hour stretch, to go into "a full-court press to understand what the failure is" and how to address it, NASA flight director Norm Knight said Friday. That set the stage for the spacewalk involving three of the space station's six-man crew.

The leak was in a cooling loop in a solar array that has leaked before, and astronauts tried to fix it in November. It's unclear whether the leak detected this week was the same one or a new issue.

The ammonia coolant for the power channel, one of eight used to supply electricity to the station, would likely have run out by late Friday morning had it not been shut down, NASA said.

"It is a serious situation, but between crew and experts on the ground, it appears to have been stabilized," Hadfield tweeted Thursday.

Still, the space station's crew -- which also includes three Russian cosmonauts -- were never in danger because of it, NASA said. Moreover, the agency has said the rest of the orbiter was otherwise operating normally.

Both Marshburn and Hadfield are scheduled to leave the space station at 7:08 p.m. ET Monday with Russian cosmonaut Roman Romanenko, and NASA said Friday that the spacewalk wouldn't affect those plans.

Cassidy and Russian cosmonauts Alexander Misurkin and Pavel Vinogradov will stay behind. They'll eventually be joined by NASA astronaut Karen Nyberg, Russian cosmonaut Fiyodor Yurchikhin and European Space Agency astronaut Luca Parmitano, who are due to launch aboard a Russian Soyuz spacecraft on May 28.